The Corrective Optics Space Telescope Axial Replacement (COSTAR), removed from Hubble during Servicing Mission 4 in 2009, was an ingenious device created to solve a famous Hubble Space Telescope problem.

Soon after
Hubble began sending images from space, scientists discovered that the
telescope's primary mirror had a flaw called spherical aberration. The
outer edge of the mirror was ground too flat by a depth of 4 microns
(roughly equal to one-fiftieth the thickness of a human hair). The flaw
resulted in images that were fuzzy because some of the light from the
objects being studied was being scattered.

After this discovery, scientists and engineers developed COSTAR, corrective
optics that functioned like eyeglasses to restore Hubble's vision.

By placing small and carefully designed
mirrors in front of the original Hubble instruments, COSTAR --installed during the 1993
First Servicing Mission -- successfully improved
their vision to their original design goals.

The
optics of the Wide Field and Planetary Camera 2, which was also installed during the 1993 mission, were designed to correct
for spherical aberration. This was the first instance of a Hubble instrument being constructed with built-in corrective optics.

All the instruments installed since were built with such internal corrections for spherical aberration, eventually making COSTAR unnecessary. During Servicing Mission 4, astronauts finally removed COSTAR to make way for a new instrument, the Cosmic Origins Spectrograph.